1. Field of the Invention
This invention is related generally to liquid-gas separating devices for catching and retaining condensable liquid materials from a flowing gas stream, and more particularly to a trap apparatus and method for removing tantalum pentaethoxide (TAETO) from the exhaust gas flowing out of a reaction/deposition chamber used for depositing thin films of tantalum pentoxide in the fabrication of electronic devices.
2. State of the Art
Tantalum pentoxide (Ta2O5) is a highly dielectric, electrically insulating, material, which is used extensively in integrated circuits and other semiconductor devices to increase charge storage density, thus to reduce cell areas required for capacitors without reducing capacitance. For example, tantalum pentoxide films are used as storage capacitor dielectric layers in high density integrated circuits, such as DRAM memory chips, and for gate dialectrics for CMOS devices.
Thin films of tantalum pentoxide are usually deposited on desired semiconductor integrated circuit substrates in chemical deposition chambers by well-known chemical vapor deposition (CVD) techniques. For example, tantalum pentaethoxide [Ta(OC2H5)5], also known as TAETO, is often used as a precursor material and reacted with oxygen or ozone in a low pressure chemical vapor deposition (LPCVD) reaction chamber to deposit tantalum pentoxide films. Typically, the liquid TAETO is vaporized and delivered into the reaction chamber through a heated, upstream delivery system. The interior of the reaction chamber is also supplied with an atmosphere of oxygen (O2) or ozone (O3) and is often maintained at a relatively high temperature, such as about 450xc2x0 C., to assist the chemical reaction between the TAETO and the oxygen (O2) or ozone (O3) to produce the desired tantalum pentoxide (Ta2O5), which deposits as a solid film on a substrate in a reaction chamber. The deposition of the tantalum pentoxide (Ta2O5) thin film is often followed by an annealing step at about 800xc2x0 C. in an oxygen atmosphere to enhance the quality of the tantalum pentoxide film.
The primary reaction of TAETO with oxygen (O2) in the reaction chamber to produce tantalum pentoxide (Ta2O5) is:                                           2            ⁢                                          Ta                ⁡                                  (                                                            OC                      2                                        ⁢                                          H                      5                                                        )                                            5                                +                      30            ⁢                          O              2                                      ⁢                  →                      450            ⁢            xc2x0            ⁢                          xe2x80x83                        ⁢                          C              .                                      ⁢                                            Ta              2                        ⁢                          O              5                                +                      20            ⁢                          CO              2                                +                      25            ⁢                          H              2                        ⁢            O                                              (        1        )            
The primary reaction of TAETO with ozone (O3) in the reaction chamber to produce tantalum pentoxide (Ta2O5) is:                                           2            ⁢                                          Ta                ⁡                                  (                                                            OC                      2                                        ⁢                                          H                      5                                                        )                                            5                                +                      20            ⁢                          O              3                                      ⁢                  →                      450            ⁢            xc2x0            ⁢                          xe2x80x83                        ⁢                          C              .                                      ⁢                                            Ta              2                        ⁢                          O              5                                +                      20            ⁢                          CO              2                                +                      25            ⁢                          H              2                        ⁢            O                                              (        2        )            
However, there are also secondary reactions involved in the deposition of Ta2O5 because of the generation of the water vapor (H2O) byproduct of the reaction in either equation (1) or equation (2) above. In fact, the rate of such secondary chemical reaction between TAETO and water is much higher than the primary chemical reaction (equation (1) or (2)) between TAETO and O2 or O3. Such secondary reaction forms tantalum pentoxide (Ta2O5) and alcohol, as follows:                                           2            ⁢                                          Ta                ⁡                                  (                                                            OC                      2                                        ⁢                                          H                      3                                                        )                                            5                                +                      5            ⁢                          H              2                        ⁢            O                          ⁢                  →                      450            ⁢            xc2x0            ⁢                          xe2x80x83                        ⁢                          C              .                                      ⁢                                            Ta              2                        ⁢                          O              5                                +                      10            ⁢                          C              2                        ⁢                          H              5                        ⁢            OH                                              (        3        )            
This secondary reaction of equation (3) in a 450xc2x0 C. reaction chamber proceeds orders of magnitude faster than the primary reaction of either equation (1) or (2), but it proceeds rapidly at ambient (room) temperature as well. Therefore, if any TAETO in the vacuum system downstream from the reaction/deposition chamber has an opportunity to react with water vapor, the resulting solid tantalum pentoxide can damage vacuum system components, such as pumps, valves, and the like.
Only about 10%-20% of the TAETO precursor is consumed in the reaction chamber to produce tantalum pentoxide in order to achieve acceptable, uniform film deposition across a wafer substrate surface. The remainder of the unused TAETO precursor passes along with the carbon dioxide (CO2) and alcohol (C2H5OH) byproducts out of the reaction chamber and into the pumping foreline, i.e., the pipe leading to the vacuum pump, of the evacuation system. Most, if not virtually all, of the water vapor byproduct of reaction (1) or (2) is consumed in reaction (3), as evidenced by there only being a very small, negligible amount of light, white coating of tantalum pentoxide on the interior wall of the foreline, close to the reaction/deposition chamber exit, in most conventional tantalum pentoxide deposition systems. Therefore, the initial problem from TAETO in the evacuation system is encountered when TAETO condenses, and then problems occur when TAETO reacts with water vapor that back streams into the vacuum system or gets into the vacuum system in another manner, such as during maintenance, to form solid tantalum pentoxide, as explained in more detail below.
First, TAETO will condense in an unheated pumping foreline (a temperature of about 125 to 150xc2x0 C. is typically required to keep the TAETO from condensing in the foreline), and liquid accumulation in the foreline will affect vacuum pump characteristics and interfere with maintenance of a desired vacuum pressure in the reaction/deposition chamber. Second, when liquid or vapor TAETO is carried into the vacuum pump, the lifetime of the pump can be reduced significantly, especially if solid tantalum pentoxide forms in the pump from reaction of TAETO and any water vapor that gets into the vacuum system. Since most, if not all of the water produced by the initial reaction (equation (1) or (2)) is consumed by the secondary reaction (equation (3)), as explained above, the problems of solid tantalum pentoxide formation and accumulation in the evacuation system, such as in the vacuum pump, valves, regulators, and the like, is usually the result of ambient moisture from the atmosphere, or from wet scrubber equipment that is often installed downstream from the vacuum pump, that back streams into the pump and other components of the vacuum system to combine with TAETO that is exhausted from the reaction/deposition chamber and thereby produces solid tantalum pentoxide in the vacuum system. Also, solid tantalum pentoxide can form very rapidly when either gaseous or liquid TAETO combines with water vapor in ambient air from the atmosphere, which makes maintenance and service of components in the vacuum system very difficult. Solid tantalum pentoxide has to be cleaned mechanically out of pipes and other components of the vacuum system. Further, TAETO is valuable and, if recovered, can be processed and purified, for re-use, e.g., as precursor for tantalum pentoxide deposition.
Conventional traps designed for universal application to remove a variety of solid CVD reaction byproducts are being used to remove TAETO in evacuation systems of tantalum pentoxide CVD reaction chambers. However, such conventional traps have very low trapping efficiency, because trapped liquid also has much higher mobility compared to trapped solid byproducts, or they require too much maintenance and down time when used to trap TAETO because of the rapid reaction with water vapor and resulting solids formation, as described above. Also, such maintenance and down time with such conventional traps is exacerbated, because, as mentioned above, solid tantalum pentoxide, which forms very rapidly when liquid TAETO in such conventional traps is exposed to water vapor in air, has to be cleaned out of such traps mechanically, or the traps would have to be replaced.
Accordingly, it is a general object of the present invention to provide a more efficient and more effective apparatus and method for trapping and removing condensable liquid products and byproducts from a flow of gas.
A more specific object of this invention is to provide a more efficient and more effective apparatus and method for trapping and removing TAETO from an evacuation system of a tantalum pentoxide CVD or LPCVD reaction chamber.
An even more specific object of this invention is to provide a trapping apparatus and method that is designed especially for the unique trapping and handling problems presented by TAETO, which is exhausted in gaseous form from a CVD or LPCVD reaction/deposition chamber and then condenses into liquid of varying viscosities that are very prone to react with water vapor to form solid tantalum pentoxide.
Additional objects, advantages, and novel features of the invention are set forth in part in the description that follows and will become apparent to those skilled in the art upon examination of the following description and figures or may be learned by practicing the invention. Further, the objects and the advantages of the invention may be realized and attained by means of the instrumentalities and in combinations particularly pointed out in the appended claims.
To achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described herein, the trapping method of this invention comprises routing a gas stream containing condensable material, such as TAETO, through a tortuous flow path in a vacuum to contact the gas stream with cooled surfaces, which enhance condensation and are configured to accommodate collection and flow of liquid condensate of varying viscosities to a reservoir in a manner that does not clog or interfere with the gas flow and that minimized re-evaporation of the collected liquid condensate back into the gas stream.
To further achieve the foregoing and other objects and in accordance with the purposes of the present invention, as embodied and broadly described herein, the trap apparatus of this invention comprises a primary trap chamber divided by a plurality of vertically spaced apart partitions into a plurality of portions or plenums disposed vertically, one over another, and that are connected together in fluid flow relationship by openings in the partitions. A secondary trap chamber is preferably surrounded by the primary trap chamber in a housing and is connected in fluid flow relationship with the last or lowest of the plurality of portions or plenums of the primary trap chamber. The primary trap chamber is preferably separated from the secondary trap chamber by an inner cylindrical wall, and it is preferably surrounded and enclosed by an outer cylindrical wall. A cooling system removes heat from surfaces in both the primary trap chamber and the secondary trap chamber. The openings in the partitions are preferably misaligned vertically to force a longer flow path and to thereby increase contact of gaseous flow with a surfaces in the primary trap chamber. A small drain opening in an end wall under the primary and secondary chambers is connected to a reservoir, which is separated from the gaseous flow path through the primary and secondary chambers, so that liquid condensate flowing off surfaces in the primary and secondary chambers and collected by the end wall can flow through the small drain opening and into the reservoir for storage remote from the gaseous flow. A valve between the small drain opening and the reservoir is provided to accommodate temporary removal of the reservoir for emptying and cleaning while maintaining the vacuum in the primary and secondary chambers and minimizing, if not preventing, any ambient water vapor from entering the primary and secondary chambers through the small drain opening.